Semiconductor device



July 19, 1966 J. N. CARMAN SEMICONDUCTOR DEVICE Original Filed Sept. 22,1959 INVENTOR JUSTICE N. CARMAN BY F 8 aMhNT UI ATTORNEY United StatesPatent Office 3,261,075 Patented July 19, 1966 3,261,075 SEMICONDUCTORDEVICE Justice N. Carman, Gloucester, Mass, assignor to CarmanLaboratories Inc, a corporation of Massachusetts Original applicationSept. 22, 1959, Ser. No. 841,513, now Patent No. 3,200,310, dated Aug.10, 1965. Divided and this application Dec. 10, 1964, Ser. No. 435,103

3 Claims. (Cl. 2925.3)

This application is a division of my application Serial No. 841,513,filed September 22, 1959 (now Patent No. 3,200,310, granted August 10,1965), which in turn was a continuation-in-part of my application SerialNo. 763,- 548, filed September 26, 1958 (now abandoned).

This invention relates to semiconductor devices such as rectifiers,transistors, and negative temperature coefficient resistances, and moreparticularly to the assembly of such devices with supporting andconductive leads. The invention provides an improved junction-typesemiconductor device comprising a semiconductor crystal bonded by abonding layer fusible only at temperatures above 700 C. to conductiveleads whose coefiicient of thermal expan sion substantially matches thatof the crystal, and which in its preferred form is provided with aprotective encapsulation (preferably glass) surrounding the edge of thecrystal and fused into contact with the assembly of leads and crystal.The resulting structure is mechanically strong, and one in which theelectrical properties of the crystal are effectively protected againstdeterioration. Additionally, the invention provides a method for makingthese new and improved semiconductor devices.

Semiconductor devices are frequently in the form of a thin crystal waferhaving one or more junctions between n and p regions of the crystal.Such junctions may be formed by physically joining n and p ty-pecrystals, or by diffusion of a donor or acceptor impurity into ahighpurity silicon, germanium, or equivalent crystal. In any case, thejunction between the n and p regions terminates at the edge bounding thecrystal faces. Heretofore it has been the practice to solder fine wireleads or other conductive terminals to opposite faces of the crystal orto regions of the crystal spaced from the junction. The crystal is thenencapsulated in a protective enclosure. The resulting device isincapable of withstanding high operating temperatures, or sudden powersurges which momentarily create high temperatures, because the solderbond between the crystal and the lead wire or other terminal is easilydestroyed at only moderately elevated temperatures. Moreover, protectiveenclosures which can be applied to crystals to which leads have beenfastened by a low melting solder are not entirely reliable forprotecting the crystal from contamination and resulting deterioration ofits electrical properties. Such enclosures also are inadequate forprotecting the crystal from vibration or other mechanical damage.

The present invention provides an improved semiconductor device in whichthe foregoing disadvantages of her tofore known semiconductor assembliesare overcome. The invention is directed particularly to semiconductordevices comprising a semiconductor crystal laterally bounded by acircumferential edge and having at least one n-p junction terminating atsaid edge, with conductive 'leads secured by a bonding layer to thefaces of said crystal on opposite sides of said junction. (The term leadis used throughout this specification to include wire leads, conductivebases, and other forms of terminal connections.) In accordance with theinvention, each of the leads has a coefficient of thermal expansionsubstantially matching that of the crystal. The bonding layer is of suchcomposition that it is fusible only at temperatures above 700 C., andpreferably above 800 C. A protective envelope surrounding thecircumferential edge of the crystal generally is provided. Preferablysuch envelope is of glass having a coefiicient of thermal expansionsubstantially matching that of the crystal, and advantageously it isfused into contact therewith and also into glassto-metal sealed relationwith the end portions of the leads adjoining the crystal.

The invention is particularly applicable to semiconductor devices inwhich the semiconductor crystal is siliconiferous, i.e. is composed ofsilicon or a semiconducting silicon compound such as silicon carbide. Inorder to protect such a crystal from deleterious contamination byimpurities from the leads, and also to protect the leads from attack byetching solutions to which they may be exposed, it is desirable to haveall surfaces of the leads adjacent the crystal provided with a thincontinuous surface coating of at least one metal selected from the groupconsisting of platinum, palladium, rhodium, iridium, ruthenium, osmium,gold, silver, and alloys thereof which fuse at temperatures above 700 C.The bonding layer by which the leads are secured to the faces of thecrystal preferably is formed of a metal selected from the groupconsisting of silver, platinum, palladium, and alloys thereof which fuseat above 700 C., bonded to a face of the crystal. With such a coatinglayer on the leads, and with such a bonding layer securing the leads tothe crystal, the crystal is easily able to withstand fusion about it ofthe protective glass envelope; and it is easily able to withstandsubsequent operation at very high temperatures (up to a red heat)without mechanical injury and without deterioration of its electricalproperties.

The method of the invention for making glass encapsulated semiconductordevices of the character described comprises bonding conductive leads tothe faces of the crystal on opposite sides of its n-p junction by meansof a bonding layer fusing at above 700 C., enclosing the circumferentialedge of the crystal with glass having a coefiicient of thermal expansionsubstantially matching that of the crystal, and heating theglass-enveloped assembly of leads and crystal to a temperature above thesoftening point of the glass until the glass has fused into directcontact with the assembly. Most conveniently, this method is carried outby enclosing the circumferential edge of the crystal and the adjoiningend portions of the leads in a tubular bead of the glass, and heatingthe head in such position to a temperature above its softening pointuntil it has fused into glass-to-metal sealed relation with the leadsand, preferably, into direct contact with the circumferential edge ofthe crystal.

Preferred and illustrative embodiments of the invention are described indetail below with reference to the ac companying drawings, in which FIG.1 is a sectional view of a junction-type diode assembly;

FIGS. 2 and 3 are enlarged fragmentary sectional views showingalternative forms of the diode of FIG. 1;

FIG. 4 is a sectional view of a transistor triode assemy;

FIG. 5 is a sectional view of a transistor triode and temperaturecompensating resistance assembly; and

FIG. 6 is a sectional view of a photodiode.

It is to be noted that the drawings are highly schematic. They show thesignificant structural features of devices made according to theinvention, but the proportions and geometrical forms of the parts ofactual devices may and often will be quite different from theproportions and geometrical forms of the corresponding parts as shown inthe drawings.

Shown in FIG. 1 is a diode assembly comprising a semiconductor crystalwafer 10, which in actual size is typically 0.002 inch (0.05 mm.) inthickness and 0.060 inch (1.5 mm.) in diameter, mounted between a pairof conductive leads 11. Advantageously the crystal 10 is pure silicon towhich minute controlled amounts of impurities have been added so as toform p and n regions on either side of a junction 12. The junctionterminates at the circumferential edge 13 of the crystal, and theconductive leads are bonded to faces of the crystal on opposite sides ofthe junction. While a number of different substances including germaniumand certain 1ntermetallic crystals, possess semiconductor properties andcan he used in preparing semiconductor devices according to theinvention, the invention particularly contemplates devices in which asiliconiferous crystal is the semiconducting element. By the termsiliconiferous I mean to include crystals of pure silicon andsemiconducting crystals of silicon compounds, such as silicon carbide,which possess chemical and metallurgical properties similar to puresilicon. The formation of siliconiferous crystal wafers having n-pjunctions therein is well understood in the art and is not in itself apart of this invention.

The conductive leads 11 bonded to the crystal comprise a core 14 of amaterial having a coefiicient of thermal expansion substantiallymatching that of the crystal 10. Silicon and most other semiconductorsare quite brittle, and are rather easily injured mechanically ifsubjected to substantial stress. To assure against injury of the crystalwhen it is heated and cooled through a large temperature range (as iscontemplated by the invention), the materials in direct contact with thecrystal should therefore have expansion characteristics which do notdiffer very much from that of the crystal. The closeness with which thethermal expansivity of the lead material must match that of the crystalis of course dependent on such factors as the actual size of thecrystal, the temperature range through which it is to be heated, therigidity of the bondbetween crystal and leads, and the geometricalconfiguration of crystal and leads. These factors must be such that themaximum thermally-induced stress to which the crystal is subjected doesnot exceed that which it is able to withstand without cracking or otherinjury. The thermal expansion coefficient of silicon is approximately4.2x cm./cm./ C., and in general relatively massive bodies having athermal expansion coefficient in the range from 3.0 to 5.5 10-' cm./cm./C. are sufficiently closely matched to permit being bonded directly tothe crystal. Tungsten, molybdenum and Kovar (an alloy composed nominallyof 29% nickel, 17% cobalt, balance iron) all are metals having thermalexpansion coefficients which are sufficiently close to that of siliconto be useful in forming the core 14 of the conductive leads. Of thesemetals, molybdenum and tungsten are preferred because their thermalexpansion characteristics most closely approximate those of silicon,

and because they have good thermal conductivity characteristics. Thermalconductivity of the lead is important, to facilitate dissipation of heatwhen the device is operated at a high power level.

For a number of reasons, the core component of the lead is encased by asubstantially continuous surface coating 15, which, as shown in FIG. 1,should thoroughly cover all surfaces of the leads adjacent the crystal.The coating 15 protects the lead core from corrosive attack by theetching solution used for the final clean-up of the surface of thecrystal prior to encapsulation, and it prevents deleterious oxides ofthe core component from impairing the electrical characteristics of thecrystal. To perform these functions, the protective coating 15 should beresistant to oxidation at elevated temperatures and to attack by theetching solution. Materials which possess these properties, and whichare well suited to form the protective coating 15, are the noble metalsplatinum, palladium, rhodium, iridium, ruthenium, osmium, gold, andsilver. Alloys of these metals, with each other and with yet othermetals, also may be employed to form the protective coating. Rhodium,however, is preferred for the reason that it is particularly effectivefor protecting the core component of the lead assembly from corrosionand oxidation, and in addition it forms a very effective diffusionbarrier between the lead and the crystal. Protection of the crystal frominterdilfusion with the lead core is of importance in the case ofdevices intended for operation at sustained high temperatures.

A thin layer of any of the aforementioned coating metals maybe appliedto the tungsten or other core component by electroplating, usingconventional plating baths and plating techniques for doing so. Thinelectroplated coatings may be somewhat porous, and in order to insurethorough coverage of the lead core by the coating, electroplatedcoatings preferably are sintered at 1000 C. to 1200 C. or higher for asubstantial period of time (usually ten minutes to an hour or so). Suchheating effects closure of any pores that may exist in theelectrodeposited metal.

Instead of applying the protective coating 15 by electrodeposition, itmay be applied by any other desired coating procedure. For example, itmay be applied by an evaporation or sputtering technique, or it may insome cases be formed by chemical reduction of the metal on the surfaceof the core component 14. An alloy coating may be formed by applyingsuccessive coatings of components of the alloy, and then heating toeffect interdiffusion of these coatings into a single coating of more orless uniform composition throughout its thickness.

It is sometimes advantageous to form a composite protective coating onthe core of the lead. Such composite coats may include a non-nobleundercoat, over which one of the above specified coating metals isapplied. For example, a relatively thick undercoat of cobalt or nickelmay first be applied to the core component, and then a thin outer coatof platinum, rhodium, or the like, may be formed over such undercoat. Inthis manner effective coating of the leads may be effected with aminimum thickness and hence minimum cost of noble metal.

The protective coating 15 preferably covers all surfaces of the corecomponent 14 adjacent the crystal 10. Such coating may be applied to allsurfaces of short lengths of the core component; or alternatively, asillustrated in FIG. 1, the core component of tungsten, molybdenum, orthe like, may be joined to a copper or other metal lead wire .16 by aweld indicated at 17, and the surfaces of the core component may then beplated up to such weld.

Each lead 11 is bonded to a face of the crystal 10 at a positionsubstantially separated from the peripheral boundary of the up junction12, by a bonding layer 18. This bonding layer is preferably formed of ametal selected from the group consisting of silver, platinum, palladiumand alloys thereof which fuse at temperatures above 700 C. This layer issecurely bonded both to a coated surface of the lead and to a face ofthe crystal, and integrally joins the leads and the crystal togetherinto a unitary structure. It also provides a low resistance electricaland thermal connection of the leads to the crystal, to permit free flowof electric current to and from the crystal, and ready dissipation ofheat from the crystal.

Silver, and more particularly an alloy composed of 5% to 15% by weightgold and the balance silver, are preferred metals for the bonding layer18. The melting temperature, the electrical and thermal conductivities,the ability to Wet silicon, and the oxidation and corrosion resistancesof these metals makes them especially suitable for bonding the leads tothe crystal. Five to fifteen percent gold alloyed with silversignificantly enhances the resistance of the metal to attack by theetching reagent; and since gold does not form intermetallic cornpoundswith silicon it does not lead to excessive hardening of the bondinglayer. Platinum and palladium alloyed (in amounts from 5% to 25% byweight) with silver are even more effective than gold for increasingresistance of the metal to attack by the etchant.

Bonding of the leads to the crystal faces is accomplished by heating thebonding metal to a fusion temperature while it is in contact with boththe leads and the crystal. For example, a thin disk of the bonding metalmay be held under pressure between the end faces of the leads and thecrystal faces to which the leads are to be joined, and the assembly maythen be heated sufliciently to effect fusion (partial or complete) ofthe bonding metal. Normally for this purpose the assembly is heated to atemperature in the range from 900 C. to 1050 C. A temperature above 900C. is preferable in order to insure the formation of a secure bondbetween the crystal and the leads; but heating above 1050 C. should beavoided in order to insure against impairment of the electricalproperties of the crystal.

Pure silver melts at 960 C., and an alloy of gold, 90% silver (apreferred bonding alloy) melts at only a slightly higher temperature.However, fusion of these bonding metals in contact with thesemiconductor crystal will occur at substantially lower temperatures.Evidently enough interdilfusion of silicon or germanium with the bondingmetal occurs to form an alloy which fuses at a temperature substantiallybelow the normal melting temperature of the bonding metal itself. Hencea fusion bond between the crystal and the lead is achieved even attemperatures below the actual melting temperature of the bonding metal.Consequently, even platinum, palladium, and alloys of these metals witheach other or with silver which normally melt at temperatures above 1050 C. can be fused to the semiconductor crystal Without heating to abovesuch temperature. The bonding metal, including its silicon eutectic,should, however, be fusible only at temperatures above 700 C., andpreferably above 800 C., so as to be able easily to withstand fusion atthe temperature to which the assembly of leads and crystal must beheated to fuse a glass envelope thereabout.

In case .an alloy bond is desired, the alloy and the bond may be formedconcurrently. For example, a bonding layer composed of a gold silveralloy may be formed by applying a coating of gold to the core of thelead, and then heating the thus-coated lead in contact with a layer ofsilver which is in turn in contact with the crystal. The gold and silverfuse together, or interdifiuse, and the silver fuses to the crystal, toform a gold-silver alloy bond between the lead and the crystal. Otheralloy bonding layers may be similarly formed. The proportions of thecomponents of the alloys thus formed may be approximately controlled bycontrolling the thicknesses of the respective layers prior to heating toform the bond.

Ordinarily heating the assembly of lead, crystal and bonding metal tothe bonding temperature is carried out in a reducing atmosphere. It mayhowever be carried out in a vacuum chamber from which the air has beenexhausted. It may also be carried out in a neutral atmosphere, such asan atmosphere of an inert gas, e.g. nitrogen, argon, helium, or thelike.

Although particular mention has been made of supplying the metal for thebonding layer 18 in the form of a thin disk, it is obvious that it canbe supplied in other ways equally well. It can for example be suppliedin the form of finely divided metal powder pressed between the crystaland the leads, or it may be supplied in the form of a plating or coatingdeposited electrolytically or by other means on the ends of the leads,or on the faces of the crystal. It may also be supplied in the form of abutton fused on to the end of the lead. Or, when the coating on thesurface of the lead components 14 is composed of one of the metalssuitable for use as a bonding metal, then such coating may itselffunction in the dual capacity of a coating and as a source of bondingmetal of which the bonding layer 18 is formed.

It is sometimes desirable to alloy the metal of the bonding layer withan element of the third or fifth group of the Periodic Table which iscapable of functioning as an acceptor or donor of electrons from or tothe semiconductor crystal. Third group elements which may be usedsuccessfully are boron, aluminum, gallium, and indium; and fifth groupelements which may be used with success are phosphorus, arsenic andantimony. Inclusion of any of these elements in the bonding layer isdetermined in accordance with desired electrical properties of thedevice. They may lower the electrical resistance of the crystal in theconducting direction, or they may otherwise modify the electricalcharacteristics of the crystal itself or of the interface between thecrystal and the bonding layer. If any of these third or fifth periodelements is alloyed with the metal forming the bonding layer, itpreferably is used in a concentration in the range from 1% to 5% byweight of the bonding metal alloy.

Having welded the leads 11 to the semiconductor crystal, the next stepordinarily is to subject the exposed peripheral edge 13 of the crystalto an etching treatment to remove any thin surface contamination whichmight permit development of an electrical shunt around the peripheralboundary of the n-p junction 12. Such etching ordinarily is performed byimmersing the assembly of leads and crystal in an oxidizing acidsolution. The manner in which such etching is carried out, and theparticular solution used for the purpose is well understood in the artof making semiconductor devices and is not in itself a part of thisinvention. However, one of the important functions performed by thecoating metal 15 on the surfaces of the leads is to protect the corecomponents 14 from attack by the etching solution. This protection isimportant not only to avoid corrosion of the leads, but moreparticularly to preclude contamination of the crystal by reactionproducts of the etchant with the metal of the lead core. The noblemetals used in accordance with the invention to form the bonding layers18 also are highly resistant to the etching solutions normally employed,and are neither corroded thereby nor converted to reaction products withthe etching solution which might objectionably contaminate thesemiconductor crystal.

A feature of the invention is that the assembly of leads and crystaladvantageously is encapsulated in a protective envelope preferably ofglass, which may be fused into direct contact with the peripheral edgeof the crystal. Such encapsulation may be effected immediately aftercompletion of the etching treatment. FIG. 1 shows a device in which aprotective glass envelope 19 is fused about the peripheral edge 13 ofthe crystal and into glassto-metal sealed relation with the adjoiningend portions of the leads 11. A convenient procedure for applying theglass envelope 19 is to insert the assembly of leads and crystal into ashort tubular glass bead, in such position that the crystal is disposedabout at the center of the bead. The assembly then is heated to abovethe softening temperature of the glass for a sufficient period of timeto fuse the glass into direct contact with the end portions of the leadsand preferably also with the edges of the crystal. If desired, the glassbead while heated to above its softening temperature may be mechanicallypressed to insure effective sealing of the glass to the leads and to theexposed edges of the crystal.

The glass of which the envelope 19 is composed should be one having acoefiicient of thermal expansion substantially matching that of thecrystal 10, and of course also substantially matching the coefficient ofthermal expansion of the lead cores 14. As with the lead cores, the coefiicient of expansion of the glass preferably falls in the range from3.0 to 5.5x l0 cm./cm./ C. A number of glasses having such a coefiicientof thermal expansion are available commercially, and others may becompounded specially. The glass forming the protective envelope 19advantageously also has a softening point well below the fusiontemperature of the bonding layer 18. While it is feasible to fuse theenvelope 19 into contact with the crystal edges and the leads at atemperature above the fusion temperature of the bonding layer 18, it isnot generally convenient to do so. Accordingly, the glass employedpreferably softens sufliciently to be fused into 7 place about thecrystal and the leads at a temperature below 850 C., and preferablybelow 800 C. In the manufacture of preferred devices according to theinvention, a glass head is fused about the crystal at a temperature inthe range from 650 C. to 800 C.

The glass must of course be one having good electrical properties andmust be free of constituents which might deleteriously contaminate thesemiconducting crystal. In general, glasses composed solely of metaloxides meet these requirements. This limitation is hardly a restrictionon the scope of the wide range of glass compositions that can beemployed, however, for most glasses nominally have such composition.Mainly it is a restriction against the use of glasses made underconditions which do not exclude substantial amounts of impurities, andit is a restriction against the use of a few special glasses to whichnon-oxidic constituents are intentionally added.

Table I lists several glass compositions which can be used successfullyto form the protective glass envelope 19. It is understood that thecompositions set forth in Table I are merely illustrative of a verylarge number of glass compositions that can be used for the purpose:

TAB LE I are bonded to faces of the crystal on opposite sides of the n-pjunction by bonding layers 28 similar to the bonding layers 18 ofFIG. 1. The peripheral edge of the crystal wafer 20 is protected by aglass envelope 29 which is generally similar to the envelope 19 ofFIG. 1. However, the glass envelope 29 of FIG. 2 is formed by applying aglass glaze to the exposed peripheral edge of the semiconductingcrystal. Such glaze may be any such of the glass compositions asdescribed above in connection with FIG. 1 but is applied to the crystaledge in the form of a powder, or a slurry of the glass powder in asuitable vehicle. After the edge of the crystal has been coated with theglaze composition in dry powdered or slurry form, the assembly is heatedsufficiently to fuse the particles of glass powder together and to forman impervious protective envelope surrounding the exposed edge of thecrystal, and, preferably, bonded to at least a small annular area of theleads where they adjoin the crystal.

A generally similar structure is shown in FIG. 3. Here a thinsemiconductor crystal wafer is bonded to leads 31. The crystal shown isa diode having a single n-p junction 32 terminating at the peripheraledge 33 of the Glass G e O 2 P20 5 K2 A pereent *Connnercial lowexpanding borosilicate glasses for glass-to-metal seals with tungsten.

While the invention contemplates fusing the glass envelope 19 intodirect contact with the peripheral edge 13 of the semiconductor crystalsuch is not essential. Even when such contact is intended the glassenvelope may fail to make contact with all points of the crystal edge.For example, the viscosity of the hot glass may be sufficiently high soas to prevent it from flowing into tiny crevices between the lead andthe crystal, or a small gas bubble may become entrapped between the edgeof the crystal and the glass envelope. Such defects ordinarily are notof significance. Semiconductor devices in which such defects exist willordinarily perform as well and display the same mechanical ruggedness asdevices in which the glass envelope intimately makes contact with theentire peripheral surface of the crystal.

It is of some importance that the glass envelope be effectively bondedin tight glass-to-metal sealed relation with the leads 11. One of thefunctions of the glass envelope is to protect the crystal from exposureto extraneous contaminants in the ambient atmosphere; and to performthis function effectively, the interface between the glass envelope andthe surfaces of the leads should be free from any leakage pathsproviding communication between the outside atmosphere and the surfaceof the crystal. Similarly, there should be no leakage paths through theglass bead providing communication from the atmosphere to the surface ofthe crystal. A leak-free seal of the glass to the leads is not nearly socritical as in a vacuum tube, however. Whereas a small leak will quicklyresult in inoperativeness of the vacuum device, it merely opens the doorto a somewhat shortened life of a semiconductor device according to thisinvention.

FIGS. 2 and 3 show alternative procedures for forming the protectiveglass envelope about the peripheral edge of the semiconductor crystal.FIG. 2 portrays schematically a diode comprising a semiconductor crystal20 bonded to leads 21. The crystal comprises a thin wafer of silicon orthe like having a n-p junction 22 terminating at a peripheral edge 23 ofthe crystal. The leads comprise a core component 24 having a coefiicientof expansion substantially matching that of the crystal 20; and thesurfaces of the core component are coated with a metal coating 25similar to the coating 15 of FIG. 1. The leads crystal. Each leadcomprises a core component 34 of metal having a thermal expansioncoefficient substantially matching that of the crystal 30, and providedwith a protective surface coating 35, similar to the coating 15 ofFIG. 1. The leads are welded to opposed faces of the crystal 30 bybonding layers 38 similar to the bonding layers 18 of FIG. 1. Aprotective glass envelope 39 is fused about the peripheral edge of thecrystal 30, in glassto-metal sealed relation with adjoining end portionsof the leads 31.

Prior to applying the protective glass envelope 39, the peripheral edgeportion of the crystal 30 is subjected to an oxidizing atmosphere at anelevated temperature to convert it to an edge layer 33a of silicondioxide (or germanium dioxide, depending on the composition of thecrystal 30). Such oxidation of the crystal edge may be effected byheating the assembly of crystal and leads after the leads have beenwelded in place and the crystaledge has been etched, in an oxidizingatmosphere. For example, the assembly may be heated to a temperature ofabout 600 C. in an atmosphere of nitrogen saturated with water vapor.The oxidized crystal boundary layer 33a provides a protective coveringfor the terminus of the n-p junction which is as free from objectionablecontaminants as is the semiconductor crystal material itself. It guardsagainst the possibility that contaminants possibly present in the outerprotective glass envelope 39 may impair the electrical performance ofthe crystal.

The simple diode structure described above with reference to FIGS 1 to 3illustrates the significant and characteristic features of semiconductordevices made in accordance with the invention. FIGS. 4- to 6 show othertypes of semiconductor devices which similarly can be made in accordancewith the invention.

FIG. 4 shows a junction transistor triode assembly made in accordancewith the invention. It comprises a semiconductor crystal wafer 40 of thep-n-p type having two p type regions separated by a pair of n-pjunctions 41, 41a from an intermediate 11 type region. One face of thewafer is bonded to a cylindrical lead 42, and the opposite face isbonded to a tubular lead 43. A central region 44 of the crystal, on theface to which the tubular lead 43 is bonded, is etched away to below thejunction 41a, to expose the intermediate 11 type region of the crystal,and a third lead 45 is bonded to this intermediate region.

Each of the leads comprises a central core 42a, 43a, 45a having acoeflicient of thermal expansion substantially matching that of thecrystal, and having its surfaces adjacent the crystal covered by aprotective coating 42b, 43b, 45b similar to the protective coating 15 ofFIG. 1. Bonding layers 46, 47 and 48, each similar to the bonding layer18 of FIG. 1, provides for secure bonding of the leads to the crystal. Aprotective envelope of glass 49 is fused about the peripheral edge ofthe crystal wafer and into glass-to-metal sealed relation with the leads42 and 43. A body of fused glass 49a also fills the annular spacebetween the tubular lead 43 and the lead 45 bonded to the intermediateregion of the crystal, to insulate these leads from each other.

A modified transistor triode having a temperature sensitive resistancein series with it is shown in FIG. 5. This device comprises asemiconducting junction transistor wafer 50 of the p-n-p type having acentral 11 type region separated by n-p junctions 51 from flanking ptype regions. An annular base support 52 is bonded by a bonding layer 53to the 11 type region of the crystal Wafer, the peripheral portion ofone of the p type regions being etched away to permit such bond-ing. Acylindrical body 54 of semiconductor material, preferably of the samesemiconductor material as the crystal wafer 50, is bonded to theprojecting face of the peripherally etched p type region by a bondinglayer 55. A lead 56 is bonded to the opposite [face of the crystal waferby a bonding layer 57, and another lead 58 is bonded to the outer end ofthe semiconductor body 54 by a bonding layer 57a.

The tubular base 52 and the leads 56 and 58 each comprise a core member52a, 56a and 58a having a coefficient of thermal expansion substantiallymatching that of the crystal. The surfaces of the base and the leadswhich are adjacent the crystal wafer are completely covered by acontinuous coating 52b, 56b and 58b similar to the protective coating 15of FIG. 1. The bonding layers 55, 57 and 57a likewise are similar to thebonding layers 18 of FIG. 1. A protective glass envelope 59, similar tothe envelope 19 of FIG. 1, is fused into contact with the peripheraledge of the crystal Wafer 50, and into glassto-metal sealed relationwith the base 52 and the oppositely disposed lead 56. A body of glass59a also is fused about the semiconductor body 54 and intoglass-to-meta-l sealed relation with the annular base 52 and the lead 58centrally disposed therein.

In this assembly, the cylindrical body 54 of semiconducting materialserves as compensating resistance for counteracting any thermally causedchange in the performance of the crystal Wafer. The ohmic value of thecompensating resistor is determined by its length, its cross section,and its resistivity (which in turn is controlled by the amount andnature of donor :or acceptor elements introduced into it). Althoughshown bonded to a p type region of the crystal wafer, the semiconductingbody 54 may equally well be bonded to an 11 type region. Instead ofbeing bonded to a central projecting face of the crystal, within theannulus of the tubular base 52, it may itself be annular in form andbonded to the etched periphery of the crystal wafer. Although FIG. showsa device in which the crystal wafer is a triode transistor, it mightequally Well be a diode or other semiconductor element. These and othermodifications in the structure shown obviously can be made withoutdeparting from the features which characterize the invention hereindescribed.

FIG. 6 shows an application of the invention to a photoresponsivesemiconductor device. The structure of FIG. 6 comprises a semiconductingcrystal diode 60 having p and n regions on opposite sides of an n-pjunction 61. An annular frame 62 is bonded to one face of the crystalwafer, and a lead 63 is bonded to the opposite face. The annular frame62, and the leads 63, each comprises a core element 62a, 63a having acoefficient of thermal expansion substantially matching that of thecrystal 60. All surfaces of both the tframe and the lead adjacent thecrystal are covered by a thin continuous coating 62b, 63b of a coatingmetal similar to that of the coating 15 of FIG. 1. The annular frame 62is bonded to the crystal wafer by an annular bonding layer 64, similarin character and composition to the bonding layers 18 of FIG. 1; and thelead 63 is similarly bonded by a bonding layer 65 to the op- .positeface of the crystal. A protective fused glass envelope 66 similar to theenvelope 19 of FIG. 1 is fused about the periphery of the crystal wafer,and into glass-tometal sealed relation with the frame 62 and the lead63.

Although not shown in FIG. 6, a transparent or translucent protectiveglass window may be fused in glass-tometal sealed relation in theaperture of the frame 62 to prevent contamination of the otherwiseexposed area of the crystal face.

It is evident from the foregoing that the basic features of theinvention may be applied to a wide variety of semiconductor devices, andto numerous different mechanical designs of such devices. In all suchdevices, the invention provides a structure in which the semiconductorcrystal is most effectively protected against contamination which mightlead to deterioration of its electrical properties, and is at the sametime incorporated in an assembly having a notably high degree ofmechanical ruggedness. Moreover, in the preferred embodiments,semiconductor devices according to the invention are capable ofsustained and reliable operations at temperatures far above those atwhich semiconductor devices heretofore known could operate at all.

I claim:

1. The method of making a glass encapsulated semiconductor devicecomprising a semiconductor crystal bounded by a circumferential edge andhaving at least one n-p junction terminating at said edge, whichcomprises bonding conductive leads to faces of said crystal on oppositesides of said junction by means of a bonding layer fusing at above 700C., enclosing the circumferential edge of the crystal with an envelopeof glass having a coefficient of thermal expansion substantiallymatching that of the crystal, and heating the glass enveloped assemblyof leads and crystal to a temperature above the softening point of theglass but below the fusing temperature of the bonding layer until theglass has fused into direct contact with the assembly.

2. The method of making a glass encapsulated semiconductor devicecomprising a siliconiferous crystal bounded by a circumferential edgeand having at least one n-p junction terminating at said edge, whichcomprises bonding conductive leads to faces of said crystal on oppositesides of said junction by means of a bonding layer fusing at above 800C., enclosing the circumferential edge of the crystal and the adjoiningend portions of the leads in a tubular bead of glass having acoefficient of thermal expansion substantially matching that of thecrystal, and heating said bead while it surrounds the crystal to atemperature above its softening point but below 800 C. until it hasfused into glass-toanetal sealed relation with said leads.

3. The method of making a semiconductor device com- :prising asiliconiferous crystal bounded by a circumferential edge and having atleast one n-p junction terminating at said edge, and having conductiveleads bonded to faces of said crystal on opposite sides of saidjunction, which comprises applying to the conductive leads a thincontinuous coating of a metal of the group consisting of platinum,palladium, rhodium, iridium, ruthenium, osmium, gold, silver, and alloysthereof, fusing a thin bonding layer of a metal selected from the groupconsisting of silver, platinum, palladium and alloys thereof to a faceof said crystal, whereby said leads become bonded to the crystal,enclosing the circumferential edge of the crystal in a protectiveenvelope of glass having a coefficient of thermal expansionsubstantially matching that of the crystal, and heating theglass-enclosed crystal to a temperature above the softening point of theglass but below the fusing temperature of the bonding layer until theglass has fused into direct contact with the assembly of leads andcrystal.

4. The method according to claim 3, characterized in that the coatingmetal is applied to the conductive leads by electrodeposition and theresulting electrodeposit is then heated to a temperature above thesintering temperature of the coating metal.

5. The method according to claim 3, characterized in that fusion of thebonding layer is effected by heating the metal forming said layer Whileit is in contact with both the crystal and the coated sunface of thelead to a temperature in the range from 850 C. to 1050 C.

6. The method according to claim 3, characterized in that thecircumferential edge of the crystal prior to en- -12 closure in theglass envelope is subjected to controlled oxidation to form theron asurface layer of silica.

7. The method according to claim 3, characterized in that enclosure ofthe circumferential edge of the crystal is effected by disposing a glassbead thereabout and about the adjoining end portions of the leads, andfusing said bead into glass-to-metal sealed relation with said leads.

8. The method according to claim 7, characterized in that the glass beadis fused at a temperature in the range from 650 C. to 800 C.

References Cited by the Examiner UNITED STATES PATENTS 3,110,080 12/1963Boyer 2925.3

RICHARD H. EANES, 111., Primary Examiner.

1. THE METHOD OF MAKING A GLASS ENCAPSULATED SEMICONDUCTOR DEVICECOMPRISING A SEMICONDUCTOR CRYSTAL BOUNDED BY A CIRCUMFERENTIAL EDGE ANDHAVING AT LEAST ONE N-P JUNCTION TERMINATING AT SAID EDGE, WHICHCOMPRISES BONDING CONDUCTIVE LEAD TO FACES OF SAID CRYSTAL ON OPPOSITESIDES OF SAID JUNCTION BY MEANS OF A BONDING LAYER FUSING AT AVOVE700*C., ENCLOSING THE CIRCUMFERENTIAL EDGE OF THE CRYSTAL WITH ANENVELOPE OF GLASS HAVING A COEFFICIENT OF THERMAL EXPANSIONSUBSTANTIALLY MATCHING THAT OF THE CRYSTAL, AND HEATING THEGLASS-ENVELOPED ASSEMBLY OF LEADS